In recent years, a sealed secondary battery (which may be hereinafter simply referred to as “secondary battery”) represented by a lithium ion secondary battery is used as a power source not only for a mobile apparatus such as a portable phone or a notebook personal computer but also for an electrically driven vehicle such as an electric automobile or a hybrid car. A cell constituting the secondary battery has a structure in which an electrode group is accommodated in an inside of a sealed outer casing, and the electrode group is constructed by winding or stacking a positive electrode and a negative electrode with a separator interposed therebetween. For example, a laminate film such as an aluminum laminate foil is used as the outer casing. Typically, a laminate film such as an aluminum laminate foil or a metal can having a cylindrical shape or a rectangular prismatic shape is used as the outer casing.
The secondary battery has a problem that, when the electrolytic solution is decomposed due to overcharging or the like, the cell swells to be deformed in accordance with rise in the internal pressure caused by the decomposition gas and, unless the charging current or the discharging current is stopped, this results in rupture of the secondary battery. Conventionally, a safety valve that is released by an internal pressure of a predetermined level or higher (for example, about 1 MPa to 2 MPa) is provided. However, the electrolytic solution is scattered to contaminate the surroundings when the safety valve is released, so that a technique capable of preventing the rupture without depending on the safety valve is desired. Also, though the danger of reaching the rupture is low in a secondary battery of an aluminum laminate film type, there is a fear that an electric apparatus or module on which the secondary battery is mounted may be broken by swelling of the cells. Due to such circumstances, there is a demand for monitoring the deformation of the secondary battery, and a technique capable of detecting the deformation with a high degree of sensitivity is needed.
Patent Document 1 discloses a monitoring apparatus in which a pressure sensor is disposed in an inside space of a safety valve of a lithium secondary battery, and the pressure detected by the sensor is displayed on a displaying device. However, this necessitates an electric wire from the pressure sensor located in the inside of the container to the displaying device located on the outside of the container. Therefore, unless a structure for sealing the surroundings of the electric wire is added, the sealed structure is hindered. Also, Patent Document 2 discloses a sealed storage battery including a pressure-sensitive electroconductive rubber that changes a resistance value thereof in accordance with rise in the internal pressure provided in a battery case. However, an electric wire from the inside to the outside of the battery case is provided, and a special structure for retaining the sealed structure is needed in the battery case.
Patent Document 3 discloses a laminate-type battery in which, in a part of a welded part in which the fringes of a laminate film are welded to each other, a part in which metal layers are in contact with each other without intervention of a resin layer is formed, and rise in the internal pressure is detected by change in the voltage value or change in the resistance value of the metal layer when that part is peeled off. However, in this battery, detection is not carried out unless the internal pressure becomes so high that the welded part is peeled off. Also, forming the part in which the resin layer is absent or the metal layer is exposed at the welded part can disadvantageously be a cause of breakdown.
Patent Document 4 discloses a tactile sensor provided with an elastomer containing a magnetic filler and a magnetism sensor that detects a change in magnetism caused by the deformation of the elastomer, and this is constructed as a pressure sensor that detects the pressure applied to the elastomer. However, it is assumed that this tactile sensor is applied to a hand or skin of a robot on which a comparatively large external force is applied, so that there may be cases in which the sensitivity is insufficient in monitoring the deformation of the sealed secondary battery such as described above. For practical use, in order to monitor the deformation of the sealed secondary battery, it is extremely important that the sensitivity of the detection is enhanced; however, a technique therefor is not disclosed.